1. Field
The present disclosure relates generally to processing images and, in particular, to registering synthetic aperture radar (SAR) images. Still more particularly, the present disclosure relates to a method and apparatus for registering synthetic aperture radar images that have been orthorectified.
2. Background
Images are used in performing various types of operations. These operations may include, for example, without limitation, object recognition, object tracking, and/or other suitable types of operations. Oftentimes, image registration is performed prior to performing these operations. Image registration is the alignment of images of a same scene generated at different times, from different viewpoints, and/or by different sensors.
Feature-based image registration is an example of one type of image registration. Feature-based registration transforms a first image of a scene such that features in the first image align with the same features in a second image of the same scene. The second image may also be referred to as a reference image or a source image.
With feature-based image registration, different types of transformation models may be used to transform the first image to align the first image with the reference image. One type of transformation model is a linear transformation. A linear transformation may include, for example, without limitation, translation, rotation, scaling, and/or other suitable types of affine transformations. An affine transformation is any transformation that preserves collinearity between points and ratios of distances between points on a line.
Feature-based image registration may be used with different types of images. These different types of images may include, for example, without limitation, visible spectrum images, optical images, infrared images, radar images, synthetic aperture radar (SAR) images, and other suitable types of images.
Typically, synthetic aperture radar images are orthographically rectified prior to performing image registration. This process may also be referred to as orthorectification. Orthorectification is the removal of geometric distortions from an image such that the scale of the image is substantially uniform. These geometric distortions may be caused by tilt of the sensor that generated the image, terrain relief, lens distortion, and/or other suitable sources of distortion. Images that have been orthographically rectified may be referred to as orthorectified images.
Feature-based image registration of orthorectified images may include using an orthographic transformation that translates and/or rotates an orthorectified image to align with a reference image. The reference image is also orthorectified. Currently-available methods for performing feature-based image registration of synthetic aperture radar images may not be as accurate as currently-available methods for feature-based image registration of visible spectrum images.
For example, a greater amount of noise may be present in synthetic aperture radar images as compared to visible spectrum images. This greater amount of noise may make the identification of features in synthetic aperture radar images less accurate as compared to the identification of features in visible spectrum images, using currently-available methods for identifying features in images. As a result, currently-available methods for feature-based image registration of synthetic aperture radar images may be less accurate than desired.
Therefore, it would be advantageous to have a method and apparatus that takes into account at least some of the issues discussed above, as well as possibly other issues.